Thomas,

You can basically use any tile outside, but definately NOT over indoor/outdoor carpet. The carpet must be pulled up and the glue MUST be scrapped off. You need a good concrete surface to set the tile too, otherwise you'll have tile popping off. You can use a good adhesive removal chemical too. Check your local home improvement store.

Think ahead: How will you treat the edge? What type of pattern might I want? Lay the tile from the front of the porch toward the house. That way any small slivers that must be cut will be cut nearest the house, it will give a better appearance with full size tiles out in the open and cut pieces against the house.